Proton conductor, method for producing proton conductor, and fuel cell

ABSTRACT

A proton conductor includes a coordination polymer having stoichiometrically metal ions, oxoanions, and proton coordinating molecules capable of undergoing protonation or deprotonation. The coordination polymer including coordination entities that are repeatedly coordinated to bond the coordination entities with one another. Each coordination entity is either a first coordination entity or a second coordination entity. The first coordination entity is one metal ion of the metal ions coordinated with either at least one oxoanion of the oxoanions or at least one proton coordinating molecule of the proton coordinating molecules. The second coordination entity is the metal ion coordinated with each of at least one oxoanion of the oxoanions and at least one proton coordinating molecule of the proton coordinating molecules. At least a part of the proton conductor is non-crystalline. The proton conductor has high ion conductivity at high temperature.

CROSS REFERENCE TO RELATED APPLICATION

This application is based on Japanese Patent Application No. 2015-150728filed on Jul. 30, 2015, the disclosure of which is incorporated hereinby reference.

TECHNICAL FIELD

The present disclosure relates to a proton conductor, a method forproducing the proton conductor, and a fuel cell.

BACKGROUND

Conventionally, an operation temperature of an electrolyte material of asolid polymer fuel cell is lower than 100° C. In the conventionalelectrolyte material, since an ion is conducted through moisture in afilm, a system for controlling the moisture in the electrolyte materialis required.

From the viewpoints of reduction of the cost of the system for the solidpolymer fuel cell and simplification of the system, an electrolytematerial that has an operation temperature equal to or higher than 100°C. and operates under a non-humidification or a low-humidificationcondition. JP 2014-116276 A (corresponding to US 2014/0011103 A1)discloses such electrolyte material.

SUMMARY

An electrolyte material is desired to have further high ionconductivity.

It is an object of the present disclosure to provide a proton conductorhaving high ion conductivity, a method for producing the protonconductor, and the fuel cell.

According to an aspect of the present disclosure, a proton conductorincludes a coordination polymer having stoichiometrically metal ions,oxoanions, and proton coordinating molecules capable of undergoingprotonation or deprotonation. The coordination polymer includingcoordination entities that are repeatedly coordinated to bond thecoordination entities with one another. Each coordination entity iseither a first coordination entity or a second coordination entity. Thefirst coordination entity is one metal ion of the metal ions coordinatedwith either at least one oxoanion of the oxoanions or at least oneproton coordinating molecule of the proton coordinating molecules. Thesecond coordination entity is the metal ion coordinated with each of atleast one oxoanion of the oxoanions and at least one proton coordinatingmolecule of the proton coordinating molecules. At least a part of theproton conductor is non-crystalline. The proton conductor according tothe aspect of the present disclosure has high ion conductivity at hightemperature (e.g., 100° C. or more).

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, features and advantages of the presentdisclosure will become more apparent from the following detaileddescription made with reference to the accompanying drawings, in whichlike parts are designated by like reference numbers and in which:

FIGS. 1A to 1D are diagrams illustrating organic polymers for examplesof an additive material;

FIG. 2 is a diagram illustrating results of x-ray diffraction in example1 and comparative example 1;

FIG. 3 is a diagram illustrating a method for producing a single cell ofa fuel cell;

FIG. 4 is a diagram illustrating a perspective view of the single cellof the fuel cell; FIG. 5 is a diagram illustrating results of x-raydiffraction in example 2 and comparative example 2; and

FIG. 6 is a flowchart diagram for illustrating a method for producing aproton conductor.

DETAILED DESCRIPTION

An embodiment of the present disclosure will be described.

1. Proton Conductor

A proton conductor of the present embodiment includes a metal ion, anoxoanion, and a proton coordinating molecule. Both of the oxoanion andthe proton coordinating molecule coordinate to the metal ion to formcoordination polymer, or, either one of the oxoanion and the protoncoordinating molecule coordinates to the metal ion to form thecoordination polymer.

For example, the oxoanion may coordinate to the metal ion in a monomerstate without condensation. In this case, high proton concentration isretained in the proton conductor, and the proton conductor showsexcellent water stability.

For example, a phosphate ion, a sulfate ion and the like may beconsidered as the oxoanion. The phosphate ion is preferable because thephosphate ion has chemical stability to hydrogen. The phosphate ion maybe a hydrogen phosphate ion having one proton coordinating to thephosphate ion, or a dihydrogen phosphate ion having two protonscoordinating to the phosphate ion.

The proton coordinating molecule of the present embodiment is a moleculethat has preferably two or more coordination sites to which the protoncoordinates. The proton coordinating molecule is held within a structureof the proton conductor by a coordinate bond with the metal ion, or byan interaction of a hydrogen bond or a coulomb bond with the oxoanion.The proton coordinating molecule does not volatile even at 100° C. ormore.

From the viewpoint of ion conductivity, imidazole, triazole,benzimidazole, benztriazole and their derivatives are preferable as theproton coordinating molecule because these molecules have coordinationsites with excellent balance of coordination and desorption of theproton.

In the present disclosure, the derivative means a molecule a part ofchemical structure of which is replaced by other atom or other atomicgroup compared to the original molecule. For example, 2-methylimidazole,2-ethylimidazole, histamine, histidine and the like are derivatives ofimidazole, in which a part of chemical structure is replaced by otheratom or other atomic group.

For example, as the proton coordinating molecule, primary amine that isexpressed by a general formula of R—NH₂, secondary amine that isexpressed by a general formula of R¹(R²)—NH, and tertiary amine that isexpressed by a general formula of R¹(R²)(R³)—N may be considered. R, R¹,R² and R³ independently represent one of alkyl group, aryl group,alicyclic hydrocarbon group and heterocyclic group.

For example, as the primary amine, lower-alkylamine such as methylamine,ethylamine, propylamine, and the like, and aromatic amine such asaniline, toluidine and the like may be considered.

For example, as the secondary amine, di-lower-alkylamine such asdiethylamine, dipropylamine and the like, and aromatic secondary aminesuch as N-methylaniline, N-methyltoluidine and the like may beconsidered.

For example, as the tertiary amine, tri-lower-alkylamine such astrimethylamine, trietheylamine and the like may be considered.

For example, as the proton coordinating molecule, carbon linear chaindiamine such as ethylenediamine, and its N-lower-alkyl derivative (e.g.,tetramethylethylenediamine) and the like may be considered.

Also, as the proton coordinating molecule, saturated cyclic amine suchas pyrrolidine, N-lower-alkylpyrrolidine (e.g., N-methyl pyrrolidine),piperidine and N-lower-alkylpiperidine (e.g., N-methyl piperidine),morpholine, N-lower-alkylmorpholine (e.g., N-methylmorpholine) may beconsidered.

Furthermore, as the proton coordinating molecule, saturated cyclicdiamine such as piperadine, N-lower-alkylpiperadine (e.g.,N,N-dimethylpiperadine), 1,4-diazabicyclo[2.2.2]octane (another name:triethylenediamine) may be considered.

Although the metal ion of the present embodiment is not especiallylimited, from the viewpoint of formation of the coordination bond withthe oxoanion and/or proton coordinating molecule, transition metal ionsin high period or main-group metal ions are preferable for the metal ionof the present embodiment. Especially, cadmium ion, manganese ion,cobalt ion, cupper ion, zinc ion, and gallium ion are preferable for themetal ion of the present embodiment.

The proton conductor of the present embodiment includes the metal ion,the oxoanion and the proton coordinating molecule. In order to form thecoordination polymer efficiently, it is preferable to combine 1 to 4moles of oxoanion and 1 to 3 moles of proton coordination molecule per 1mole of metal ion.

When less than 1 mole of the oxoanion and the proton coordinatingmolecule is combined, there is a possibility that the coordinationpolymer is not formed. When more than 4 moles of oxoanion or more than 3moles of proton coordinating molecule are combined, there is apossibility that proton conductor is not formed in a solid state, theproton conductor shows high hygroscopicity, and morphological stabilityof the proton conductor is significantly decreased.

At least a part of the proton conductor of the present embodiment isnon-crystalline. Entire part of the proton conductor of the presentembodiment may be non-crystalline. The proton conductor of the presentembodiment may be a mixture of a non-crystalline part and a crystallinepart. It is confirmed that the part of the proton conductor isnon-crystalline by an x-ray diffraction.

The proton conductor of the present embodiment may include an additivematerial in addition to the metal ion, oxoanion, and the protoncoordinating molecule. For example, as the additive material, one ormore selected from a group consisting of metal oxide, organic polymerand alkali metal ion may be considered. When the proton conductorincludes the additive material, ion conductivity is improved at lowtemperature (e.g., lower than 100° C.), without lowering the performanceof the proton conductor at high temperature (e.g., 100° C. or more).

An additive amount of the additive material is preferably within a rangeof 1 to 20 parts by weight (pts. wt.) when the total weight of the metalion, oxoanion and the proton coordinating molecule is set to 100 pts.wt. When the additive material is the metal oxide or the organicpolymer, the additive amount of the additive material is preferablywithin a range of 5 to 20 pts. wt. When the additive amount is withinsuch a range, ion conductivity is improved at low temperature (e.g.,lower than 100° C.), without lowering the performance of the protonconductor at high temperature (e.g., 100° C. or more).

For example, as the metal oxide, one or more selected from a groupconsisting of SiO₂, TiO₂, Al₂O₃, WO₃, MoO₃, ZrO₂ and V₂O₅ may beconsidered.

When the metal oxide described above is employed, ion conductivity isimproved at low temperature (e.g., lower than 100° C.), without loweringthe performance of the proton conductor at high temperature (e.g., 100°C. or more). A particle diameter of the metal oxide is preferably withina range from 5 to 500 nm. When the particle diameter of the metal oxideis within such a range, ion conductivity is improved at low temperature(e.g., lower than 100° C.), without lowering the performance of theproton conductor at high temperature (e.g., 100° C. or more). Theparticle diameter of the present embodiment means a value that isobtained by taking an image of the metal oxide particle by an electronmicroscope (e.g., scanning electron microscope: SEM) and by analyzingthe image.

The organic polymer preferably has an acidic functional group. When theorganic polymer having the acidic functional group, ion conductivity isfurther improved at low temperature (e.g., lower than 100° C.), withoutlowering the performance of the proton conductor at high temperature(e.g., 100° C. or more).

For example, as the acidic functional group, carboxyl group (—COOH),sulfo group (—SO₃H) and phosphono group (—PO₃H₂) may be considered. PHof the organic polymer is preferably within a range equal to or lessthan 4.

When the pH of the organic polymer is within such a range, ionconductivity is further improved at low temperature (e.g., lower than100° C.), without lowering the performance of the proton conductor athigh temperature (e.g., 100° C. or more).

For example, as the organic polymer, polyacrylic acid (PAA) as shown inFIG. 1A, polyvinylphosphonic acid (PVPA) as shown in FIG. 1B,polystyrenesulfonic acid (PSSA) as shown in FIG. 1C and deoxyribonucleicacid (DNA) as shown in FIG. 1D may be considered.

For example, as the alkali metal ion, one or more metal ion selectedfrom a group consisting of Li, Na, K, Rb, Cs may be considered. Whensuch alkali metal ion is employed, ion conductivity is further improvedat low temperature (e.g., lower than 100° C.), and at high temperature(e.g., 100° C. or more).

2. Method for Producing Proton Conductor

When the proton conductor does not include the additive material, theproton conductor of the present embodiment is produced by mixing andstirring metal compound as metal ion source (e.g., metal oxide), oxoacid(e.g., oxoanion) and the proton coordinating molecule, and by applyingmechanical energy. For example, as shown in FIG. 6, the method forproducing the proton conductor includes a step of mixing and stirringthe metal compound, the oxoacid and the proton coordinating molecule(S60), and a step of applying the mechanical energy (S61).

In the step of mixing and stirring, solvent that dissolves or equallydisperses each of row materials may be employed. However, from theviewpoint of production cost, it is preferable to perform the step ofmixing and stirring in solvent-free condition. When the method forproducing the proton conductor is performed at a temperature higher than200° C., there is a possibility that condensation of the oxoanion (e.g.,phosphate ion) occurs.

Therefore, it is preferable to perform the method for producing theproton conductor at a temperature equal to or lower than 200° C.

The step of applying the mechanical energy is a step of applyingpressure to the raw materials (i.e., metal compound, oxoacid (oxoanion)and proton coordinating molecule). For example, the step of applying themechanical energy is performed by using a ball mill or a press machine.

When the proton conductor includes the additive material, the protonconductor of the present embodiment is produced by mixing and stirringmetal compound as metal ion source (e.g., metal oxide), oxoacid (e.g.,oxoanion), the proton coordinating molecule and the additive material,and by applying mechanical energy. In the step of mixing and stirring,it is preferable to mix and stir all of the raw materials at once.

In the step of mixing and stirring, solvent that dissolves or equallydisperses each of row materials may be employed. However, from theviewpoint of production cost, it is preferable to perform the step ofmixing and stirring in solvent-free condition. When the method forproducing the proton conductor is performed at a temperature higher than200° C., there is a possibility that condensation of the oxoanion (e.g.,phosphate ion) occurs. Therefore, it is preferable to perform the methodfor producing the proton conductor at a temperature equal to or lowerthan 200° C.

The step of applying the mechanical energy is a step of applyingpressure to the raw materials (i.e., metal compound, oxoacid (oxoanion),proton coordinating molecule and additive material). For example, thestep of applying the mechanical energy is performed by using a ball millor a press machine.

3. Fuel Cell

A fuel cell of the present embodiment employs the proton conductordescribed above as an electrolyte. In the proton conductor providing theelectrolyte of the fuel cell of the present embodiment, water is notused as medium of ion conduction. Therefore, the fuel cell of thepresent embodiment operates under the non-humidification or thelow-humidification condition. The system for controlling the moisture inthe electrolyte is not essential for the fuel cell of the presentembodiment.

Furthermore, in the proton conductor providing the electrolyte of thefuel cell of the present embodiment, liquid (including other than water)is not used as medium of ion conduction. Therefore, in the fuel cell ofthe present embodiment, it is restricted a possibility that the liquidleaks, that deterioration of the fuel cell is caused by the leakedliquid reacting at an electrode, and that an output of the fuel cell islowered by mixed potential.

EXAMPLE 1

266 mg of cadmium acetate dihydrate, 134 pi of 85% phosphoric acidaqueous solution, 138 mg of 1, 2, 4-triazole are measured and put into amortar, and thereafter, mixed and stirred for 15 minutes at roomtemperature under atmosphere environment. The mixture is dried for 15hours at 80° C. in a thermostatic chamber to obtain white powder.

The obtained white powder is analyzed by X-ray diffraction analysis. Asa result, the obtained white powder has a crystal structure in whichfour molecules of 1,2,4-triazole coordinate to cadmium ion having sixcoordination sites and two molecules of phosphate ion (H₂PO₄) coordinateto the cadmium ion in the vertical direction.

150 mg of the obtained crystalline white powder and 3560 mg of ballhaving 10 mm of diameter are enclosed in a container made of zirconiaand having 20 mL of capacity, and air in the container is substituted byargon. Thereafter, ball milling is performed for 500 minutes at 400 rpmof rotation number to obtain non-crystalline powder. It is confirmedthat the obtained powder is non-crystalline by the x-ray diffraction.The result of the x-ray diffraction is shown in FIG. 2. The obtainednon-crystalline power is one example of a proton conductor at least apart of which is non-crystalline.

As shown in FIG. 3, the obtained non-crystalline powder is molded into apellet having 10 mm of diameter to obtain an electrolyte 1. As shown byarrows A1 and A2 of FIG. 3, electrodes 3 and 5 made of gold and having10 mm of diameter are attached at both sides of the electrolyte 1 toobtain a single cell 7 shown in FIG. 4. Ion conductivity of the singlecell 7 is calculated by an AC impedance measurement. The measurement isexecuted under nitrogen atmosphere. Frequency domain is set from 0.1 Hzto 1 MHz and voltage magnitude is set to 300 mV. As a result, the ionconductivity of the single cell 7 is 1.0×10⁻⁴ S/cm at 130° C.

EXAMPLE 2

245 mg of manganese acetate tetrahydrate, 134 μL of 85% phosphoric acidaqueous solution, 138 mg of 1, 2, 4-triazole are measured and put into amortar, and thereafter, mixed and stirred for 15 minutes at roomtemperature under atmosphere environment. The mixture is dried for 15hours at 80° C. in a thermostatic chamber to obtain white powder.

The obtained white powder is analyzed by X-ray diffraction analysis. Asa result, the obtained white powder has a crystal structure in whichfour molecules of 1,2,4-triazole coordinate to manganese ion having sixcoordination sites and two molecules of phosphate ion (H₂PO₄) coordinateto the manganese ion in the vertical direction.

150 mg of the obtained crystalline white powder and 3560 mg of ballhaving 10 mm of diameter are enclosed in a container made of zirconiaand having 20 mL of capacity, and air in the container is substituted byargon. Thereafter, ball milling is performed for 500 minutes at 400 rpmof rotation number to obtain non-crystalline powder. It is confirmedthat the obtained powder is non-crystalline by the x-ray diffraction.The result of the x-ray diffraction is shown in FIG. 5. The obtainednon-crystalline power is another one example of a proton conductor atleast a part of which is non-crystalline.

As shown in FIG. 3, the obtained non-crystalline powder is molded into apellet having 10 mm of diameter to obtain an electrolyte 1. As shown byarrows A1 and A2 of FIG. 3, electrodes 3, 5 made of gold and having 10mm of diameter are attached at both sides of the electrolyte 1 to obtaina single cell 7 shown in FIG. 4. Ion conductivity of the single cell 7is calculated by an AC impedance measurement. The measurement isexecuted under nitrogen atmosphere. Frequency domain is set from 0.1 Hzto 1 MHz and voltage magnitude is set to 300 mV. As a result, the ionconductivity of the single cell 7 is 1.2×10 ⁻⁸ S/cm at 110° C.

Comparative Example 1

266 mg of cadmium acetate dihydrate, 134 pi of 85% phosphoric acidaqueous solution, 138 mg of 1, 2, 4-triazole are measured and put into amortar, and thereafter, mixed and stirred for 15 minutes at roomtemperature under atmosphere environment. The mixture is dried for 15hours at 80° C. in a thermostatic chamber to obtain white powder.

The obtained white powder is analyzed by X-ray diffraction analysis. Asa result, the obtained white powder has a crystal structure in whichfour molecules of 1,2,4-triazole coordinate to cadmium ion having sixcoordination sites and two molecules of phosphate ion (H₂PO₄) coordinateto the cadmium ion in the vertical direction.

Similarly to the example 1, a single cell is formed using the obtainedwhite powder. Ion conductivity of the single cell is calculated by an ACimpedance measurement. As a result, the ion conductivity of the singlecell is 6.3×10⁻⁸ S/cm at 130° C.

Comparative Example 2

245 mg of manganese acetate tetrahydrate, 134 μL of 85% phosphoric acidaqueous solution, 138 mg of 1, 2, 4-triazole are measured and put into amortar, and thereafter, mixed and stirred for 15 minutes at roomtemperature under atmosphere environment. The mixture is dried for 15hours at 80° C. in a thermostatic chamber to obtain white powder.

The obtained white powder is analyzed by X-ray diffraction analysis. Asa result, the obtained white powder has a crystal structure in whichfour molecules of 1,2,4-triazole coordinate to manganese ion having sixcoordination sites and two molecules of phosphate ion (H₂PO₄) coordinateto the manganese ion in the vertical direction.

Similarly to the example 1, a single cell is formed using the obtainedwhite powder. Ion conductivity of the single cell is calculated by an ACimpedance measurement. As a result, the ion conductivity of the singlecell is 8.3×10⁻¹⁰ S/cm at 110° C.

Although the embodiment of the present disclosure is described, thepresent disclosure is not limited to the embodiment and may beimplemented in various other ways.

For example, a function of one of the elements of the embodiment may bedispersed in plural elements, or functions of the plural elements may becombined in the one of the elements. A part of elements of theembodiment may be omitted. At least one of the elements of the aboveembodiments may be added to the other embodiments, or at least one ofthe elements of the above embodiments may be replaced in the otherembodiments.

The present disclosure may be implemented in various ways other than theproton conductor described above, such as the electrolyte materialincluding the proton conductor and the method for producing theelectrolyte material.

While only the selected exemplary embodiment and examples have beenchosen to illustrate the present disclosure, it will be apparent tothose skilled in the art from this disclosure that various changes andmodifications may be made therein without departing from the scope ofthe disclosure as defined in the appended claims. Furthermore, theforegoing description of the exemplary embodiment and examples accordingto the present disclosure is provided for illustration only, and not forthe purpose of limiting the disclosure as defined by the appended claimsand their equivalents.

What is claimed is:
 1. A proton conductor comprising: a coordinationpolymer having stoichiometrically: a plurality of metal ions; aplurality of oxoanions; and a plurality of proton coordinating moleculescapable of undergoing protonation or deprotonation, the coordinationpolymer including a plurality of coordination entities that arerepeatedly coordinated to bond the coordination entities with oneanother, wherein: each coordination entity of the plurality ofcoordination entities is either a first coordination entity or a secondcoordination entity; the first coordination entity is one metal ion ofthe plurality of metal ions coordinated with either (i) at least oneoxoanion of the plurality of oxoanions or (ii) at least one protoncoordinating molecule of the plurality of proton coordinating molecules;the second coordination entity is the metal ion coordinated with each of(i) at least one oxoanion of the plurality of oxoanions and (ii) atleast one proton coordinating molecule of the plurality of protoncoordinating molecules; and at least a part of the proton conductor isnon-crystalline.
 2. The proton conductor according to claim 1, wherein:the plurality of metal ions are one or more selected from a groupconsisting of cadmium ion, manganese ion and cobalt ion.
 3. The protonconductor according to claim 1, wherein: the plurality of oxoanions areone or more selected from a group consisting of phosphate ion, hydrogenphosphate ion and dihydrogen phosphate ion.
 4. The proton conductoraccording to claim 1, wherein: the plurality of proton coordinatingmolecules are one or more selected from a group consisting of imidazole,triazole, benzimidazole, benztriazole and derivatives thereof.
 5. Theproton conductor according to claim 1, wherein: the plurality of protoncoordinating molecules are one or more selected from a group consistingof primary amine that is expressed by a general formula of R—NH₂,secondary amine that is expressed by a general formula of R¹(R²)—NH,tertiary amine that is expressed by a general formula of R¹(R²)(R³)—N,diamine with linear carbon chain, saturated cyclic amine and saturatedcyclic diamine, in which R, R¹, R² and R³ independently represent one ofalkyl group, aryl group, alicyclic hydrocarbon group and heterocyclicgroup.
 6. The proton conductor according to claim 1, further comprising:an additive material that is one or more selected from a groupconsisting of metal oxide, organic polymer and alkali metal ion.
 7. Amethod for producing the proton conductor according to claim 1comprising: mixing a metal compound including the plurality of metalions, the plurality of oxoanions and the plurality of protoncoordinating molecules at a temperature equal to or lower than 200° C.;and applying a mechanical power to the metal compound, the plurality ofoxoanions and the plurality of proton coordinating molecule.
 8. Themethod for producing the proton conductor according to claim 7, wherein:the mechanical power is applied by one of a ball mill and a pressmachine.
 9. A method for producing the proton conductor according toclaim 6 comprising: mixing a metal compound including the plurality ofmetal ions, the plurality of oxoanions, the plurality of protoncoordinating molecules and the additive material at a temperature equalto or lower than 200° C.; and applying a mechanical power to the metalcompound, the plurality of oxoanions, the plurality of protoncoordinating molecules and the additive material.
 10. The method forproducing the proton conductor according to claim 9, wherein: themechanical power is applied by one of a ball mill and a press machine.11. A fuel cell comprising an electrolyte including the proton conductoraccording to claim 1.